1. Rationale for Choice of the Femoral Artery in Vascular Access
The common femoral artery is the primary entry site for interventional vascular access. The majority of procedures involving interventions upon major arteries, including those of the limbs, neck, viscera, heart and head, are performed through needle entry into the common femoral artery. In the vast majority of cases, certainly greater than 95% in the USA, needle entry into the common femoral artery is done via a retrograde stick (i.e., a needle enters the artery in a direction opposite the flow of blood).
Antegrade stick (i.e., a needle enters the artery in the same direction as the flow of blood), in which the operator stands on the patient's left facing the feet, is rarely employed. There are several reasons for the facts stated above.
The first reason is operator-based. The stance and posture of a retrograde approach to the common femoral artery are quite natural. Most persons (85-92%) are right-hand dominant. An operator standing at the supine patient's right groin and facing towards the patient's head will find an ideal ergonomic position for right hand maneuvers involving reach, grasp, pinch, push-pull, pronation and supination of the hand and wrist. The operator's natural range of motion of the combined finger, wrist, and elbow joints very comfortably blankets a work area centered upon the groin.
The second reason is target size. The common femoral artery (CFA) lumen diameter has been extensively studied in health and disease states, and in a patient population typically provides a minimum lumen diameter of 4 mm to 6 mm. In many patients the lumen diameter reaches 8 to 10 mm. Catheter bores for common vascular interventions typically range from 6 French (diameter 2 mm) to 8 French (diameter 2.7 mm). The CFA thus easily accommodates the outside diameters of tubular instrumentation.
Length of the target vessel is also important, as the approach angle of the needle determines potential tip placement at each depth. In 200 angiographic measurements the mean common femoral artery length was 43.3 mm, and it was given as 22.5 to 50 mm in 75% of a large number of direct measurements.
The above explains why the CFA is frequently chosen as a target. It must be explained, however, why the retrograde rather than the antegrade stick route is the predominant choice. FIGS. 1A, 1B, and 1C show the target segment of the common femoral artery (CFA). An internal view of Body 3 is shown in FIG. 1A depicting CFA 1 (shown in solid lines), Inguinal Ligament 4, Profunda Femoris Artery (PFA) 7 (shown in dotted lines), Superficial Femoral Artery (SFA) 8 (shown in dashed lines), Anterior Superior Iliac Spine (ASIS) 9, Right Femur 10, Femoral Head 11, and Coccyx 12. One reason the retrograde rather than the antegrade stick route is the predominant choice is the longer Target Segment 2 length of CFA 1 when Needle 6 approaches retrograde (FIG. 1B) rather than Target Segment 2′ of CFA 1 when Needle 6 approaches antegrade (FIG. 1C). This is due to the multiple topographic curvatures of Body 3 as well as obstruction by the Inguinal Ligament 4 and the Abdominal Protuberance 5. Additionally, both the topographic window for needle entry into the skin, and the Swath 13, 13′ (the conical three-dimensional zone through which Needle 6 may pass in order that its tip will strike Target Segment 2, 2′) are also smaller in the antegrade stick approach (FIG. 1C) to CFA 1 as compared with the traditional retrograde pathway (FIG. 1B).
Body habitus is frequently abnormal in patients undergoing treatment for vascular disease. In the retrograde stick approach to CFA 1, operators have long been comforted in their use of a traditional retrograde needle placement by the fact that the approach angle and swath of the needle pathway are not materially altered by patient bulk (see FIG. 2A with normal Body 3 and Abdominal Protuberance 5 and FIG. 2B with abnormal Body 3′ and Abdominal Protuberance 5′). Longer needles may be needed to traverse the thicker body wall, but the approach angle and Swath 13 for access need not change.
In the antegrade stick technique, however, body habitus substantially narrows Swath 13′ of potential needle passage (see FIGS. 3A and 3B). Because CFA 1 Target Segment 2′ is also reduced in length, the technical challenges in the antegrade approach to a large patient are often insurmountable.
The ideal approach angle for a needle entering CFA 1 is between 30 and 45 degrees. If much steeper, 60 to 90 degrees, the subsequent placement of larger bore devices will lead to crimping or, worse, laceration (“cheese-wire” effect) of the arterial wall, with hemorrhage. In almost no cases of antegrade approach to CFA 1 is the ideal angle not blocked by Inguinal Ligament 4 and other structures superior to the groin (see FIG. 1).
A third technical hindrance to catheterization of SFA 8 via an antegrade directed needle stick is the problem of the Wire-Extrusion Vector 14 (see FIG. 4A). Operators have long experienced the ease with which the retrograde stick approach places a wire almost unfailingly in the iliac system. This is because of the unique spacial positioning of the needle tip aimed retrograde. Because of the natural approach angle which matches with the direction of CFA 1 as it passes under Inguinal Ligament 4 and becomes the posterior-directed External Iliac Artery (EIA) 38 (see FIG. 9), extrusion of the wire from Needle Bore 15 is virtually always aimed in the right direction.
In the case of an antegrade directed needle stick, the opposite is true. The mandatory vertical and posterior aim of Needle Bore 15 and Wire-Extrusion Vector 14′ almost always ensures that the wire will be extruded in the direction of PFA 7, instead of entering the SFA 8. Because Swath 13′ for Needle 6 approach is so narrow in the antegrade technique, the needle tip itself can move through only a very small swing angle as the operator attempts to correct its aim, misdirecting the wire into the PFA 7 (see FIGS. 4B and 4C).
The fourth reason is control and closure of the arteriotomy. Intentional entry into SFA 8 for placement of larger (5 French or greater) devices, is problematic. At the origin of SFA 8 from CFA 1 the diameter of the artery drops precipitously to a lumen diameter of less than 5 mm, as flow divides from CFA 1 into two substantial branch channels, SFA 8 and PFA 7. SFA 8 is not only smaller in diameter but possesses decreased arterial wall strength and integrity in comparison to CFA 1. Surgeons will often find the SFA 8 wall friable and unforgiving when it is sutured, a problem compounded by the artery's smaller lumen. SFA 8 is therefore avoided whenever possible as a site in which to originate a bypass graft, with CFA 1's stronger and larger structure being preferred. FIG. 5A shows an 8 French (diameter 2.7 mm) Sheath 16 entering into a CFA 1 having a Lumen 17 of 6 mm in diameter and a Wall Thickness 18 of 2 mm. Also shown for comparison is 8 French Sheath 16 entering into a SFA 8 having a Lumen 19 of 4 mm in diameter and a Wall Thickness 20 of 1.5 mm.
The catheter interventionalist placing a sheath in an artery faces an additional problem. The tubular mass inserted creates a roughly circular arteriotomy corresponding to the outside diameter of that sheath. This arteriotomy must then be closed in some way, i.e., sealed, once the sheath is removed. 8 French Sheath 16 inserted into CFA 1 produces an arteriotomy which occupies much less of a percentage circumference of the vessel than in SFA 8 (see FIG. 5B). Compared to CFA 1, an 8 French arteriotomy in SFA 8 produces a much larger break in the circular integrity of SFA 8, allowing Lumen 19 to gape when 8 French Sheath 16 is removed. Given two tubes, one larger and one smaller, a slit of the same length made transversely in each will disrupt shape-retention properties and tubular integrity much more in the smaller than in the larger tube. For this reason, SFA 8 more frequently demonstrates bleeding or disruption when entered with large bore devices.
Studies have shown as much as a 10% rate of pseudoaneurysm formation when sizeable catheters are deliberately placed into SFA 8. This is likely due in part to the difficulty in compressing SFA 8 manually after sheath removal. CFA 1 can be compressed by fingertip pressure on the skin overlying the puncture site, because the round bony surface of the Femoral Head 11 lies immediately beneath (see FIG. 1). SFA 8 has no corresponding bone structure deep to it which would allow effective manual compression. In the final analysis, the safest and most certain pathway for a large-bore catheter into the vascular tree is via an arteriotomy in the CFA 1.
2. Anatomy of the Femoral Artery
Anatomy of the femoral zone is complex and can be deceiving to the unschooled. CFA 1 lies in a depression, Femoral Triangle 21, seen immediately below the fold of the groin (see FIG. 6). Emerging from beneath Inguinal Ligament 4 as it leaves the pelvic cavity, CFA 1 (not visible in FIG. 6) enters the thigh at a point equidistant from ASIS 9 and the pubic symphysis (not shown in FIG. 6). CFA 1 is a continuation of a large artery—the EIA 38 (see FIG. 9). The vessel simply changes names to become CFA 1 as it crosses beneath Inguinal Ligament 4.
In the upper thigh, CFA 1 resides between the Femoral Vein 22 medially and the femoral nerve laterally (not shown in FIG. 6), in a triangular space with distinct boundaries. Superiorly is Inguinal Ligament 4; laterally, Sartorius Muscle 23; and medially, Adductor Longus Muscle 24. Deep to the femoral artery, separating it from the spherical Femoral Head 11, is the psoas major tendon (not shown in FIG. 6). Superficial to the femoral artery, forming a roof over Femoral Triangle 21 in the upper thigh is the Fascia Lata 25.
3. Topography of Femoral Artery Access
Body-surface planes and curvatures in the femoral depression tend to prohibit an antegrade approach. The femoral arteries (CFA 1, PFA 7, and SFA 8) reside in the femoral triangle concavity. Access to the femoral branches is affected by the depth of that depression, as well as the other compound curvatures of the abdomen, pelvis, pubis and thighs (see FIG. 7). In smaller and thinner persons, the curvatures are still present but may be less pronounced. But in heavier bulkier individuals the mounding and angulation of tissue can present formidable obstacles.
There are four prominent topographic curvatures shown in FIG. 7. Abdominal Protuberance 5 is the abdominopelvic protuberance, sometimes exaggerated as a pannus, containing the muscular abdominal wall, and fatty tissue, which if large, may also include the anterior peritoneum containing the small intestine and even the colon. Transverse Groove 26 is the furrow or crease formed where the inguinal canal meets the upper thigh. Muscular Curvature 27 is the mound of medial and lateral musculature bounding the depression of the Femoral Triangle 21. Sub-Pubic Pit 28 is the empty space defined by the confluence of the pubis and inner thighs.
4. Known Difficulties of the Antegrade Approach
Antegrade access is not widely touted in the literature, nor utilized extensively, due to its technical difficulty. For the foregoing reasons, medical authors have repeatedly cautioned against the antegrade approach. Dr. Giuseppe Biondi-Zoccai recommends a minimum caseload of 60 antegrade procedures to assure competency. Dr. Schneider noted that even the easier, retrograde approach resulted in less than optimal needle placement in 56% of cases, including 13% entirely beyond the borders of CFA 1. Dr. Schneider advocates against a routine antegrade approach. Dr. Narins emphasizes the steeper learning curve and increased risk of vascular complications with antegrade stick of the common femoral artery.
As a result of these and other problems, operators have not embraced antegrade femoral access. Interventionalists have instead relied upon the safety and practicality of the retrograde up-and-over technique: to reach the right leg, stick the left common femoral artery; for the left leg, stick the right common femoral artery. Nonetheless, there are enormous advantages to be gained from the antegrade approach.
5. Impetus to Develop a Safe and Easy Antegrade Approach to the Femoral Artery
Antegrade placement and manipulation of endovascular treatment devices is the most promising frontier for treatment of lower extremity arterial disease. Manufactured devices for precise work in the lower extremities—particularly if utilized to treat targets below the knee—tend to be difficult to maneuver when working over the distances and past the multiple twisting turns involved in the retrograde up-and-over access technique.
In the up-and-over method a catheter which enters the right common femoral artery retrograde must immediately track deep posteriorly following the external iliac artery down into the pelvis along the sacrum. Then it must rise abruptly within the common iliac artery, turning sharply towards the midline. The catheter then crosses the aortic bifurcation at an angle greater than 270 degrees. Another set of acute angles ensues as the catheter backtracks through the pelvis repeating the iliac course and curvatures in reverse. It will then emerge beneath the inguinal ligament, cross the “speed-bump” of the contralateral common femoral artery and its branches. At this point the catheter must be maneuvered along a steadily narrowing pathway in the superficial femoral artery until it reaches another s-curve, this time in the anterior-posterior plane, as it enters the popliteal artery and traverses the knee. Thereafter lie three successive sharp-angled take-offs of arterial branches whose diameter is now less than 3 mm, less if badly diseased.
To accomplish this, catheters must be longer. However, the increased length sacrifices pushability and control. Tight atherosclerotic plaques must be crossed by pushing in the opposite direction of catheter path at the target. This is not only mechanically disadvantageous, but requires “opposite-think” and 3-dimensional conceptual efforts which are not always easy for an operator. As a result, widespread application of certain devices has been limited by difficulty in controlling the catheters at distant lesions. Because of the predominant pattern of retrograde femoral access, manufacturers have been forced to compromise device control for length, and performance has suffered. Effective therapeutic devices which function optimally in the antegrade direction have thus been hindered in reaching a patient population which could benefit by their use.
There are natural advantages to the right-hand dominant operator which accrue when standing at the patient's right groin facing the head. In antegrade access to the legs these advantages are also in full play. Once antegrade access is established, the operator stands at the supine patient's left hip. So positioned, maneuvers of the operator's hands are directed towards the target vessels, along the axis of the catheter system. This affords all the mechanical and spacial advantages with which operators are familiar with in traditional retrograde access to the upper body.
A solution to these difficulties in access to the leg arteries would be the development of a process which makes antegrade access easy, safe, and routine. The technique should have a short learning curve, and should utilize device configurations with which the operator is already familiar. Ideally it should be performed in the operator position and via the anatomic approach most familiar to practitioners. The REAP procedure and associated devices described below are designed to provide such a solution.